Stormwater

Pervious pavement naturally absorbent

A porous pavement primer

Pervious pavement is a design alternative that allows water to percolate through the pavement structure and into underlying soils—an achievement that directly conflicts with even lecture on traditional pavement design delivered in universities across the country. Yet this unconventional approach, used in some regions for the past 25 years, is proving to be successful in meeting stormwater management challenges and regulations. Across the nation, more developers are proposing porous pavement, and the resulting benefits are reason enough for public works officials to take a closer look at its use.

Benefits of porous pavement include decreased hydroplaning and glare, increased visibility and traction, and elimination of a stormwater collection system and detention basin area. This pavement technology is approved by the U.S. Environmental Protection Agency as a best management practice, and has been proven to reduce the rate of runoff and levels of associated pollutants in addition to increasing the ground water recharge rate.

These benefits are achieved with the use of an innovative design. Pervious pavement, also referred to as porous or permeable pavement, is actually a system consisting of an open-graded surface layer of concrete, asphalt, or pavers, typically placed over an open-graded aggregate base layer. The pavement structure has a large percentage of void spaces allowing water to permeate through the surface and base at a rate comparable to that of a grass lawn. Depending on the stormwater collection needs of a particular design, water reaching the base is either allowed to seep into the subgrade or is collected in a system of perforated pipes lying within the open-graded base and then conveyed to a discharge point.

Although the cost to build a porous pavement may initially seem to be more than that of a traditional pavement, this added cost is offset by the savings resulting from the elimination of a stormwater collection system and detention basin.

REVIEWING APPLICABILITY

Pervious pavement can be used for roadway construction, subdivision development, and public or private site development. Past installations indicate that applications across the country have been primarily in the construction of parking lots. However, this method also has been extended successfully to sidewalks, bike paths, and playground surfaces. Applications not suited for porous pavement are areas subjected to high traffic volumes or speeds. The California Stormwater Best Management Practice Handbook states that this pavement type "should be limited to highways with low traffic volumes, axle loads, and speeds (less than 30 mph); car parking areas; and other lightly trafficked or non-trafficked areas.”

Although permeable pavers have been used to construct roads in subdivisions in several areas of the country, the use of porous pavement for roadway construction has not been overwhelmingly adopted by a large number of communities, most likely due to traffic volume and speed constraints. In cold weather regions, this hesitation could also be related to concerns over performance during freeze-thaw cycles.

In May 2004, the National Ready Mixed Concrete Association (NRMCA) released a report, “Freeze-Thaw Resistance of Pervious Concrete,” which offers recommendations for use of pervious concrete in areas experiencing cycles of freezing and thawing. This report groups cold weather regions into three types and offers guidelines for construction of pervious concrete for each.

Dry freeze and hard dry freeze—With little precipitation throughout the winter, these areas experience 15 or more freeze-thaw cycles in a year and are typically located in the higher elevations of the western United States. The use of pervious concrete pavement in these areas is acceptable provided the concrete is placed over a 4- to 8-inch thick layer of clean aggregate base.

Wet freeze—This group describes areas experiencing precipitation throughout the winter accompanied by 15 or more freeze-thaw cycles—typically located in the eastern United States. Again, pervious pavement can be used in these locations provided the concrete is placed over a 4- to 8-inch thick layer of clean aggregate base.

Hard wet freeze— Areas in this group experience below freezing temperatures for an extended length of time causing the ground to remain continuously frozen throughout much of winter. In order to successfully implement a pervious concrete pavement system in these areas, the report recommends placement of an 8- to 24-inch layer of clean aggregate base below the concrete, addition of an air-entraining admixture to the pervious pavement mix, and a perforated pipe system in the base to carry water away from the pavement.